Adenoviruses have been proposed as suitable vehicles to deliver genes to a host. There are a number of features of adenoviruses that make them particularly useful for the development of gene-transfer vectors for human gene therapy.
The adenovirus genome is well characterized. It consists of a linear double-stranded DNA molecule of approximately 36000 base pairs (“bp”). The adenovirus DNA contains identical Inverted Terminal Repeats (“ITRs”) of approximately 90-140 base pairs with the exact length depending on the serotype. The viral origins of replication are within the ITRs exactly at the genome ends.
The biology of the adenoviruses is characterized in detail. The adenovirus is not associated with severe human pathology in immuno-competent individuals. The virus is extremely efficient in introducing its DNA into a host cell; the virus can infect a wide variety of cells and has a broad host-range. The virus can be produced at high virus titers in large quantities.
The virus can be rendered replication defective by deletion of the early-region 1 (E1) of the viral genome (Brody et al., 1994). Most adenoviral vectors currently used in gene therapy have a deletion in the E1 region, where desired genetic information can be substituted.
Based on these features, preferred methods for in vivo gene transfer into human target cells make use of adenoviral vectors as gene delivery vehicles. However, drawbacks associated with the therapeutic use of adenoviral vectors in humans still exist. A major drawback is the existence of widespread pre-existing immunity among the population against adenoviruses. Exposure to wild-type adenoviruses is very common in humans, as has been documented extensively (reviewed in Wadell, 1984). This exposure has resulted in immune responses against most types of adenoviruses, not alone against adenoviruses to which individuals have actually been exposed, but also against adenoviruses which have similar (neutralizing) epitopes. This phenomenon of pre-existing antibodies in humans, in combination with a strong secondary humoral and cellular immune response against the virus, can seriously affect gene transfer using recombinant adenoviral vectors.
To date, six different subgroups of human adenoviruses have been proposed which in total encompasses 51 distinct adenovirus serotypes. (See, Table 1.) A serotype is defined on the basis of its immunological distinctiveness as determined by quantitative neutralization with animal antisera (horse, rabbit). If neutralization shows a certain degree of cross-reaction between two viruses, distinctiveness of serotype is assumed if A) the hemagglutinins are unrelated, as shown by lack of cross-reaction on hemagglutination-inhibition, or B) substantial biophysical/biochemical differences in DNA exist (Francki et al., 1991). The nine serotypes identified last (42-51) were isolated for the first time from HIV-infected patients (Hierholzer et al., 1988; Schnurr et al., 1993). For reasons not well understood, most of such immune-compromised patients shed adenoviruses that were rarely or never isolated from immune-competent individuals (Hierholzer et al., 1988, 1992; Khoo et al., 1995, De Jong et al., 1998).
The vast majority of people have had previous exposure to adenoviruses, especially the well-investigated adenovirus serotypes 5 and type 2 (“Ad5” and “Ad2”) or immunologically related serotypes. Importantly, these two serotypes are also the most extensively studied for use in human gene therapy.
As previously stated, the usefulness of these adenoviruses or cross-immunizing adenoviruses to prepare gene delivery vehicles may be seriously hampered, since the individual to whom the gene delivery vehicle is provided, will raise a neutralizing response to such a vehicle before long.
Thus a need exists in the field of gene therapy to provide gene delivery vehicles, preferably based on adenoviruses, which do not encounter pre-existing immunity and/or which are capable of avoiding or diminishing neutralizing antibody responses.